Theoretical investigation on the ground- and excited-state properties of novel octupolar oligothiophene-functionalized truxenes and dipolar analogs

The density functional theory (DFT) is used to compute the ground-state geometries, ionization potentials (IPs), electron affinities (EAs) and HOMO–LUMO gaps (ΔEH–L) of novel octupolar oligothiophene-functionalized truxenes and the corresponding dipolar analogs. The lowest singlet excited-state geometries of some oligomers have been investigated by the singles configuration interaction (CIS) method. The absorption and emission spectra are calculated by time-dependent DFT (TDDFT) on the basis of the ground- and excited-state geometries, respectively. Our calculations are in good agreement with the available experimental results. The calculated results show that all oligomers are nonplanar in their ground electronic states. The structural relaxation upon excitation results in the planarity of dipolar oligomers and one branch of octupolar oligomers. Because fluorene and truxene segments in two series are not the repeated units of oligomers, the chain dependence of IPs, EAs, ΔEH–L and excitation energies do not follow the usual linear 1/n rule. We have used three plotted points closest to polymer to plot lines; and extrapolated the resultant linear relationship to infinite chain length; and finally obtained the relative properties of polymers.